1. Technical Field of the Invention
The present invention relates to stators for dynamoelectric machines that are used in, for example, motor vehicles as electric motors and electric generators, and methods of manufacturing the stators.
2. Description of the Related Art
A dynamoelectric machine, such as an electric motor and an electric generator, generally includes a rotor, a stator, and a housing that accommodates both the rotor and the stator. The rotor is fixed on a rotating shaft that is rotatably supported by the housing. The stator is fixed in the housing so as to surround the radially outer periphery of the rotor. Moreover, the stator includes a hollow cylindrical stator core and a stator coil mounted on the stator core. The stator core has a plurality of slots that are formed in a radially inner surface of the stator core and spaced in the circumferential direction of the stator core at predetermined intervals. The stator coil is made up of a plurality of electric wires mounted on the stator core. Each of the electric wires includes a plurality of in-slot portions, which are received in the slots of the stator core, and a plurality of connecting portions that are located outside of the slots to connect the in-slot portions.
The above stator coil can be formed with a flat band-shaped electric wire assembly that is manufactured by a conventional method disclosed in, for example, Japanese Patent First Publication No. 2004-104841. According to the conventional method, a plurality of electric wires are first formed to have a triangular-wave shape. Then, one of the electric wires is held stationary, and the other electric wires are sequentially woven onto the stationary electric wire to form the flat band-shaped electric wire assembly. More specifically, in the weaving step, each of the other electric wires is repeatedly rotated about its axis by 90° and moved toward the stationary electric wire by a half of its pitch.
Further, the flat electric wire assembly may be rolled by a predetermined number of turns to form a hollow cylindrical electric wire assembly. After that, a plurality of stator core pieces may be mounted to the hollow cylindrical electric wire assembly. Then, the stator core pieces may be joined together, forming the stator in which the hollow cylindrical electric wire assembly makes up the stator coil and the stator core pieces together make up the stator core.
In the stator obtained in such a manner as described above, the axial end portions of the stator coil, which are made up of the connecting portions of the electric wires, protrude from the axial end faces of the stator core. Hereinafter, the axial end portions of the stator coil will be referred to as coil ends of the stator coil. Since each of the connecting portions of the electric wires extends to form the shape of a triangle, the axial end faces of the coil ends become uneven in the extending directions of the connecting portions. Moreover, each of the connecting portions of the electric wires has an apex which is axially furthest in the connecting portion from the stator core. For each radially-adjacent pair of the connecting portions of the electric wires, the apexes of the connecting portions are circumferentially spaced away from each other by the distance between each circumferentially-adjacent pair of the slots of the stator core. Consequently, the apexes of all the connecting portions of the electric wires together form a plurality of curved ridges, each of which extends obliquely from the radially inner side to the radially outer side of the stator coil, with a plurality of valleys formed between the ridges.
Moreover, during operation of the dynamoelectric machine, the temperature of the stator coil increases due to the electric current flowing therethrough, thereby causing the electric resistance of the stator coil to increase. Therefore, to suppress the increase in the electric resistance of the stator coil, a coolant (e.g., ATF) is supplied to flow along the surfaces of the coil ends and the stator core, thereby cooling the stator coil and the stator core. In addition, as shown in FIG. 18, the coolant flows, with rotation of the rotor of the dynamoelectric machine, from the radially inner side to the radially outer side of the stator core 60A, thereby cooling the axial end faces of the coil ends of the stator coil 70A.
However, with the stator coil formed with the flat band-shaped electric wire assembly manufactured by the conventional method, the flow of the coolant passing the axial end face of one of the coil ends is different from that passing the axial end face of the other coil end.
More specifically, as shown in FIG. 19A, for one of the coil ends of the stator coil 70A, the extending directions of the ridges 700A, which are made up of the apexes of the connecting portions of the electric wires, are coincident with the rotating direction b of the rotor 40A and thus also coincident with the flow direction of the coolant. It should be noted that for the sake of simplicity, only one of the ridges 700A is indicated with a solid line in FIG. 19A. Consequently, the coolant can smoothly flow along the axial end face of the coil end from the radially inner side to the radially outer side of the stator core 60A, thereby effectively cooling the coil end.
In comparison, as shown in FIG. 19B, for the other coil end, the extending directions of the ridges 700A are transverse to the rotating direction b of the rotor 40A and thus also transverse to the flow direction of the coolant. It should be noted that for the sake of simplicity, only one of the ridges 700A is indicated with a solid line in FIG. 19B. Consequently, the coolant cannot smoothly flow along the axial end face of the other coil end, thereby failing to effectively cool the other coil end.